There are a variety of reasons to stretch polymer films. Stretching can enhance or generate desired mechanical, optical, and other film properties. For example, polymer films can be stretched to provide a desired degree of uniaxial or near uniaxial orientation in optical properties. In general, perfect uniaxial orientation of a birefringent polymer results in a film (or layers of a film) in which the index of refraction in two of three orthogonal directions is the same (for example, the width (W) and thickness (T) direction of a film, as illustrated in FIG. 4). The index of refraction in the third direction (for example, along the length (L) direction of the film) is different from the indices of refraction in the other two directions. Typically, perfect uniaxial orientation is not required and some degree of deviation from the optimal conditions can be allowed depending on a variety of factors including the end-use application of the polymer film.
In optical applications, a uniaxially oriented film can provide useful optical properties such as more uniform performance across a variety of different viewing angles. Other applications can also benefit from uniaxial or near uniaxial orientation of a polymer film. For example, uniaxially oriented films are more easily fibrillated or torn along the orientation direction.